I think it is better to be a concerned about our birds rather than not be worried at all. You are doing the best you can. I am not an expert, as I have said, so I can't say for sure if your boy is a bit younger than you were led to believe or not. I can only go on what I am seeing and your description.
I have come across babies that were younger than they were thought. Breeders can mistake which baby is which, when they take them to hand raise. Its not a common occurrence but I have seen it. Strange things do happen.
My advice is centred around what you need to do, now that you have him. I agreed with
mythic55 that he does need baby food. Taking him back to demand feeding makes sense. He needs it, due to his age, what his little system can process or perhaps he is mentally not ready. All are good reasons to give him a helping hand.
Some say its part of weaning to let them beg to be fed and not comply. I don't agree. They know what they need and an extra few weeks of feeding can make all the difference. As to weight, he is a baby and been through a lot. Also he is growing, moving about etc. You say he is eating some fruit. That is great. Give him more time. Offer fruit/veg in between feeds when he indicates he is hungry to see if he will give it a try.
You could try finely sieving a thumbnail of steamed pumpkin, (no lumps) to mix with some baby food and offering just a spoonful of that mixture
once a day to start as an example. Make it fresh each time. Then his baby food as normal for the rest of the day. Then include an easy to digest fruit pulp/baby mash mix at some point. Again offering as many adult foods to play with is important. Give it another week but definitely add new things, often. Oh, is he defecating normally? Keep an eye on it. Too runny, not often are signs to look for.
@Miss Annamarie
Make sure to weigh your baby in the morning before breakfast after the first BM.
I never trust bird brokers like JC- I just enjoy the one on one with a breeder that I can trust and have legal backing if something goes wrong (Im definately not a type for forgiveness when people are knowingly irresponsible). I would put serious money on his age (ill get some photos together to prove it to you from my network, telling ekkies age when young is actually pretty easy- might be worth posting as a sticky section).
Keep up what you are doing, but watch the weight. Offer as much variety as possible, as often as possible
Yes! His main favourite fruit is mangos and blueberries! Thank you for the reassurance, my anxiety was zooming.